In the field of magnetic recording and reproducing apparatuses like magnetic disk apparatuses and magnetic tape apparatuses, an increase in the recording density has created a demand for an improvement of the performance of magnetic heads such as recording heads and reproduction heads, particularly in reading information recorded on a magnetic recording medium.
More specifically, with an increase in the coercive force of the magnetic recording medium, a material with a high saturation magnetic flux density is required for the recording head. With regard to the reproduction head, an attempt for increasing the reproduction output has been made by the use of a so-called MR (magnetoresistive effect) head utilizing the magnetoresistive effect instead of a conventional induction type head. The reason for this is to deal with the problem of a lowering of the relative velocity of the magnetic head to the magnetic recording medium, which arises with a decrease in the size of the magnetic recording medium. The magnetoresistive effect is an effect of changing the electrical resistance of a material according to a change in the external magnetic field.
As a material exhibiting such a magnetoresistive effect, magnetic thin films made of NiFe or NiCo, for example, Permalloy, are known. The MR ratio of these thin films is around 2 to 3% for NiFe, and has a maximum value of around 6% for NiCo.
The magnetoresistive effect of such a magnetic thin film is produced by the spin-orbit interaction, and depends on the angle between the direction of a measuring current and the magnetization direction of the magnetic thin film. Such a magnetoresistive effect is usually called an anisotropic magnetoresistive effect (AMR). In order to further improve the recording density of the medium, it is necessary to develop a new material having a higher MR ratio than the material showing the AMR.
In recent years, a phenomenon called a giant magnetoresistive effect (GMR), which exhibits the magnetoresistive effect based on a principle that differs from the principle of the AMR, was observed and has become the focus of attention. One example of a film having a structure showing the GMR (hereinafter referred to as the "GMR film") is a multilayer film formed by layering some ten magnetic layers and nonmagnetic layers by turns.
As illustrated in FIG. 20, the multilayer film has a multilayer structure produced by alternately depositing, for example, a thin magnetic film 32 like Co and a nonmagnetic layer 32 like Cu. In the multilayer film, adjacent magnetic layers 31 are coupled by a strong antiferromagnetic coupling. Therefore, when an external magnetic field is not present, the magnetization directions of the magnetic layers 31 are antiparallel (differ from each other by 180.degree. ). On the other hand, when an external magnetic field is applied, the directions of magnetization of the magnetic layers 31 are all aligned with the direction of the magnetic field, and thus become parallel.
The electrical resistance of the multilayer film shows a resistance change in proportion to the cosine of the angle between the magnetization directions of the magnetic layers 31 which are placed one upon another with a single nonmagnetic layer 32 therebetween. The reason for this is that the scattering of the conduction electrons varies according to the angle between these magnetic layers 31. Namely, when the magnetizations of adjacent magnetic layers 31 are antiparallel, the scattering of the conduction electrons increases, and the resistance of the film becomes a maximum. On the other hand, when the magnetizations of these magnetic layers 31 are parallel, the scattering of the conduction electrons decreases, and the resistance of the film becomes a minimum.
The MR ratio of the multilayer film is greater than that of the AMR by at least a one-digit scale. In a Co/Cu multilayer film produced by materials showing a maximum resistance change at present, an MR ratio of not lower than 60% is achieved even at ordinary temperature.
As described above, the GMR film of a multilayer structure like the multilayer film shown in FIG. 20 has an extremely high MR ratio. However, in this film, the coupling between the magnetic layers 31 is too strong. Therefore, in order to achieve a high MR ratio with this film, an extremely large magnetic field ranging from several hundreds Oe to several kOe (1 Oe=79.6 A/m) needs to be applied.
The reason for this is that the antiparallel state of the magnetizations without an external magnetic field can be achieved by the use of the exchange interaction between the magnetic layers 31. The coupling between the interacting magnetic layers 31 is extremely strong. Therefore, in order to bring the magnetizations of the magnetic layers 31 into a parallel state by cutting off the exchange interaction, an extremely high external magnetic field is required.
Thus, the use of this film in the magnetic head and the like is not practical because this film has a low sensitivity to a weak external magnetic field (the ratio of change in the magnetoresistance to a change in the external magnetic field). As a result, the MR ratio within an operative magnetic field range of the magnetic head becomes smaller.
Then, in place of the multilayer film, a spin-valve magnetoresistive effect film (hereinafter referred to as the "spin-valve film") was invented so as to achieve an improvement of the sensitivity. The spin-valve film has an antiferromagnetic layer/magnetic layer/nonmagnetic layer/magnetic layer structure. In this structure, the magnetization of one of the magnetic layers is fixed in one direction by using the exchange coupling between this magnetic layer and the antiferromagnetic layer. On the other hand, the magnetization of the other magnetic layer freely rotates according to the external magnetic field. By using a thin film made of a magnetic material having high softness like NiFe as the latter magnetic layer, the sensitivity can be improved. In the following description, the magnetic layer having a fixed magnetization direction is called the "pinned magnetization layer", and the magnetic layer whose magnetization direction freely rotates is called the "free magnetization layer".
For example, such a spin-valve film is disclosed in Japanese Publication for Unexamined Patent Application No. 358310/1992 (Tokukaihei 4-358310). FIG. 21 shows a schematic cross sectional structure of the spin-valve film. As illustrated in FIG. 21, a pinned magnetization layer 43 and a free magnetization film 41 are layered with a nonmagnetic layer 42 therebetween. Moreover, an antiferromagnetic layer 44 of, for example, manganese iron (FeMn), is deposited next to the pinned magnetization layer 43. Thus, an exchange bias produced by the exchange interaction between the pinned magnetization layer 43 and the antiferromagnetic layer 44 is applied to the pinned magnetization layer 43. As a result, the magnetization of the pinned magnetization layer 43 is fixed in one direction.
On the other hand, the free magnetization layer 41 is deposited so that the angle between the magnetization direction of the free magnetization layer 41 and that of the pinned magnetization layer 43 is 90.degree. when the external magnetic field is zero. By changing the direction of the external magnetic field, the magnetization direction of the free magnetization layer 41 can be freely changed.
Therefore, when an external magnetic field is applied to the above-mentioned film in the direction of the magnetization of the pinned magnetization layer 43, the magnetization directions of the two magnetic layers 41 and 43 become parallel. On the other hand, when the external magnetic field is applied to the film in a direction opposite to the magnetization direction of the pinned magnetization layer 43, the magnetization directions of the two magnetic layers 41 and 43 become antiparallel. Consequently, similarly to the multilayer type magnetoresistive effect film, it is possible to obtain a magnetoresistive effect which depends on the cosine of the angle between the magnetization directions of the two layers with the spin-valve film.
The spin-valve film is a so-called non-coupled type magnetoresistive effect film having no antiferromagnetic coupling between the two magnetic layers 41 and 43. In this structure, the thicknesses of the magnetic layers 41 and 43 can be increased. It is therefore possible to increase the sensitivity of resistance change with respect to the external magnetic field. Considering the above-mentioned advantages, it would be said that the spin-valve film is one of the most practical GMR films.
It has also been known that the GMR is obtained by a sandwich type magnetoresistive effect film using no antiferromagnetic film (hereinafter referred to as the "sandwich film") typified by a Co/Cu/Co film. The structure of the sandwich film is substantially the same as the spin-valve film in theory. FIG. 22 is a cross sectional view showing a schematic structure of the sandwich film. As illustrated in FIG. 22, the sandwich film is constructed by layering a pinned magnetization layer 54 and a free magnetization film 52 on a substrate 51 with a nonmagnetic layer 53 therebetween. Both of the pinned magnetization layer 54 and the free magnetization film 52 are formed by Co.
A Co oxide layer (not shown) is formed by natural oxidation on the surface of the pinned magnetization layer 54. The coercive force of the pinned magnetization layer 54 is increased by the Co oxide layer, and the magnetization direction of the pinned magnetization layer 54 is fixed in the initial magnetized direction within the operating magnetic field range. On the other hand, the magnetization of the free magnetization layer 52 which is not oxidized can freely rotate. Therefore, similarly to the above-mentioned spin-valve film, the angle between the magnetization directions of the two magnetic layers 52 and 54 of the sandwich film is varied by the external magnetic field, thereby producing the magnetoresistive effect. In this case, an antiferromagnetic coupling is not present between the two magnetic layers 52 and 54. It is thus possible to increase the thicknesses of the magnetic layers 52 and 54, and enhance the sensitivity of resistance change with respect to the external magnetic field.
As described above, it has been reported that the GMR is exhibited by a so-called non-coupled structure which uses a thin film of Co or NiFe for magnetic layers, and the difference in coercive force between two magnetic layers, instead of using the antiferromagnetic coupling between the magnetic layers.
The following description will explain the prior arts relating to the non-coupled spin-valve film and sandwich film.
1 Document (1), "Magnetization and Magnetoresistance of Co/Cu Layered Films", IEEE Transactions on Magnetics, Vol. 28, No. 5, 1992, discloses a structure of a so-called sandwich film having a very simple Co/Cu/Co layered structure. A Co oxide film is formed by natural oxidation on one of the Co surfaces of this film.
2 Document (2), "Effectiveness of Antiferromagnetic Oxide Exchange for Sandwich Layers", IEEE Transactions on Magnetics, Vol. 29, No. 6, 1993, discloses a structure in which a Co oxide film is formed on a Co/Cu/Co sandwich film by reactive sputtering using a 10% oxygen gas. It is considered that the Co oxide film is CoO which is an antiferromagnetic material whose Neel point is in the vicinity of room temperature (290 K).
It is reported that the magnetoresistive effect is produced at extremely low temperatures in the sandwich films of Documents (1) and (2). In such a sandwich film, it is considered that the exchange coupling between CoO and Co occurs when the surface of the top Co layer is oxidized or when CoO is formed by reactive sputtering. The coercive force of the Co layer adjacent to CoO is increased by the antiferromagnetic alignment, thereby producing a difference between the coercive forces of the magnetic layers sandwiching the nonmagnetic layer therebetween. Thus, the GMR is produced by these films.
3 Document (3), "Magnetoresistive Effect of Co/Cu/Co Sandwich Film", Transactions of the Japanese Applied Magnetics Society, Vol. 18, No. 2, 1994, discloses a structure in which a Co/Cu/Co sandwich film is formed on a buffer layer made of Fe or NiFe, and a Co oxide film is formed on the surface of the Co layer by natural oxidation. In this structure, the magnetoresistive effect is obtained by producing a difference in the saturation field between the two magnetic layers and by achieving the antiparallel magnetizations with the formation of the Co oxide film. The saturation field of the magnetic layer is the strength of an external magnetic field which must be applied to switch the direction of magnetization of the pinned magnetic layer. This document reports that the GMR as high as 6 to 15% is obtained even at room temperature by the use of the buffer layer made of Fe or NiFe.
4 Document (4), "The Effect of Buffer Layer in the Magnetoresistive Effect of Co/Cu/Co Sandwich Film", Transactions of Japanese Applied Magnetics Society, Vol. 19, No. 2, 1995, discloses a structure in which a Co/Cu/Co sandwich film is formed on a buffer layer made of Fe or NiFe like Document (3).
The spin-valve film shown in FIG. 21 and the sandwich film shown in FIG. 22 are non-coupled type magnetoresistive effect films. Therefore, if the coupling between the magnetic layers (interlayer coupling) is strong, the magnetization directions of the two layers do not become antiparallel, causing a problem that the GMR is not obtained. Namely, in order to obtain the GMR by achieving an antiparallel state of the magnetization directions of the two layers, it is necessary to weaken the coupling between the magnetic layers. Document (4) focused on this problem, and reported a method of weakening the coupling between the magnetic layers by forming a Fe or NiFe layer as the buffer layer of the Co/Cu/Co sandwich film.
5 Document (5), Japanese Publication for Unexamined Patent Application No. 66033/1995 (Tokukaihei 7-66033), discloses a substrate/buffer layer/Co/Cu/Co structure or a substrate/buffer layer/Co/cu/co/cap layer structure. In this structure, CoO as an antiferromagnetic material is used for the buffer layer or the cap layer. The magnetoresistive effect is improved by fixing the magnetization of the adjacent Co layer with the CoO.
With the prior arts of 2 and 5, the magnetization of the Co layer is fixed by layering CoO on the Co layer as the pinned magnetization layer by reactive sputtering. However, the Neel point of such an antiferromagnetic material, CoO, is not high. Consequently, the exchange coupled magnetic field for fixing the magnetization of the Co layer is weak at room temperature, and the antiparallel state of the magnetization directions cannot be achieved. Namely, there is a problem that the magnetoresistive effect is only obtained at low temperatures.
With the prior arts of 1, 3 and 4, the surface of the Co layer as the pinned magnetization layer is subjected to natural oxidation. However, since the natural oxide film (i.e., the film oxidized by air) is used, it is not easy to control the magnetic properties of the pinned magnetization layer. Moreover, since the saturation field of the pinned magnetization layer is decreased, the pinned magnetization layer is not stable with respect to the external magnetic field. In particular, in a structure using NiFe as the free magnetization layer for improving the sensitivity, the saturation field of the pinned magnetization layer is so small and the maximum saturation field is 16000 A/m (200 Oe). Thus, this structure is not practicable.
Further, there are reports on the buffer layer of the Co/Cu/Co sandwich film in the above-mentioned prior art documents, 3 to 5. However, Document (4) mentioned in 4 describes that the GMR is not obtained with a Co/Cu/Co structure having no buffer layer. Namely, the MR ratio depends greatly on the material and thickness of the buffer layer. In order to obtain a high MR ratio, it is necessary to form a buffer layer with a thickness of around 70 .ANG. when Fe is used, or around 100 .ANG. when NiFe is used. Thus, in the method of weakening the coupling between the magnetic layers by providing the Fe or NiFe layer as the buffer layer of the sandwich film, the effect cannot be obtained unless the Fe or NiFe buffer layer has a very large thickness. Thus, the overall resistance of such a sandwich film is much lower compared to a film having no buffer layer. Consequently, a high reproduction output cannot be obtained.
In order to obtain a high reproduction output, it is necessary to achieve a high MR ratio without lowering the resistance of the film. However, since the specific resistivity of Fe and NiFe is as small as 20 to 30 .mu..OMEGA.cm, if Fe or NiFe is used as the buffer layer, the resistance of the film is lowered. Moreover, the film structure is limited because the buffer layer is essential. Thus, there is a problem that the degree of freedom in designing the structure of a magnetoresistive effect device is small.
Japanese Publication for Unexamined Patent Application No. 65329/1995 (Tokukaihei 7-65329) discloses a sandwich structure using a high coercive force film instead of the antiferromagnetic layer. In this structure, a change in resistance is produced with a low external magnetic field by using an alloy of Co--Pt or other alloys, such as Co--Cr and Co--Ta, containing Co as a main component, as the high coercive force film.
However, the MR ratio obtained by the structure disclosed in this publication is so low as on the order of 3%. The reason for this is that since the Co alloy normally has a specific resistivity as low as some ten .mu..OMEGA.cm, the separation of the current to the Co alloy layer occurs. Furthermore, in this structure, in order to increase the MR ratio, it is considered to decrease the thickness of the Co--Pt film as thin as possible or reduce the Pt composition. However, both of these methods cause a lowering of the coercive force.